Switchgear enclosures are commonly employed in electrical power distribution systems for enclosing circuit breakers and switching equipment associated with the distribution system. The circuit breakers are used to interrupt electric power in response to hazardous current overloads in a protected circuit.
In addition to current overloads, the switchgear enclosures may also encounter hazardous conditions known as primary arcing faults Primary arcing faults are typically caused by corroded, worn or aged wiring, loose connections, overloading, lightning strikes, etc. Especially in medium or high-voltage power distribution systems, the ionized gas associated with primary arcing faults may be released at pressures and temperatures sufficient to severely damage or destroy the switchgear equipment and/or cause severe injuries to operating personnel. In addition to the ionized gases, there is a large release of incident light energy (lumens) which could be detrimental to personnel. This release of light energy is referred to as “arc flash.”
To counter or minimize effects of primary arcing faults, some present switchgear enclosures are provided with an arcing fault protection system for rapidly shorting and/or grounding the source bus of an electrical distribution system in response to the detection of an arcing fault in the enclosure. This type of arcing fault protection system includes a mechanical switch, a solid-state switch or a hybrid device of both of these types of switches, which quickly diverts current carried on the source bus to a low impedance path, thus effectively extinguishing the primary arcing fault currents present in the distribution system. A low impedance path could be a short to ground or to a three-phase short. An example of a primary arcing fault protection system is an arc terminator described in U.S. Pat. No. 5,933,308 to Garzon, titled “Arcing Fault Protection System For A Switchgear Enclosure.”
The damage done to the switchgear during an arcing fault event is a function of the speed at which the arc can be extinguished. One problem with arc terminators of the type identified above is that the operating time can be relatively slow, e.g., about 5 milliseconds. During that time, potential damage to the equipment and personnel can occur.
IEEE 1584 shows methods of calculating arc flash energies using time as one of the variables. Prior testing, including tests done in conjunction with U.S. Pat. No. 5,933,308 to Garzon, titled “Arcing Fault Protection System For A Switchgear Enclosure,” indicates that equipment damage is prevented when the time to shunt the primary arcing fault is less than 5 milliseconds.
Another type of arcing fault protection system uses an ablative plasma gun with gap electrodes on opposite ends of a chamber of ablative material. The gun injects ablative plasma into a gap between electrodes, reducing the gap impedance sufficiently to initiate a secondary arc between the electrodes that quickly absorbs energy from the primary arc fault and activates a circuit breaker. This quickly extinguishes the primary arcing fault and protects the electrical circuit. This device develops an arc in gas, which limits its lifespan, especially for high-voltage applications, by the progressive loss of insulation properties. A more detailed explanation of this type of device is described in U.S. Patent Application Publication No. 2008/0253040 to Asokan et al., titled “Ablative Plasma Gun”. In general, the use of either mechanical, or electrical, or chemical or other means, or a combination of these, to create an electrical short circuit (low impedance path) between terminals to provide an alternate current conduction path is referred to as “crowbarring.”
Although the operating time of the “arcing crowbar” device described immediately above is relatively faster than the operating time of a completely mechanical crowbar device arc terminator, one problem associated with this “arcing crowbar” device is that it fails to include a redundant system as a back-up. Another problem with this ablative type of “arcing crowbar” device is that it has a limited lifespan, wherein material essential for operation of the device is lost each time the gun fires. Furthermore, as essential material is lost, the impedance of the arc diverting path may increase over time, with the number of operations. Consequently, the likelihood increases that the path of lowest impedance may shift back to the primary arcing fault which, essentially, could cause the “arcing crowbar” device to fail.
What is needed, therefore, is a high-speed switch for an arcing fault protection system that addresses the above-stated and other problems.